1. Field of the Invention
The present invention relates to the fiber optical components used in telecommunications. More particularity, the present invention discloses a variable optical attenuator (VOA) which is able to control the amount of light power propagating in optical networks.
2. Description of the Prior Art
The industry of fiber optical communications has already proven to be indispensable for the achievement of low noise, long distance telecommunication with a heretofore-unrealizable high bandwidth. Within an optical communication network a Variable Optical Attenuator (VOA) is an important basic component with the function of controlling the propagated level of light power, such as a single-channel VOA or a VOA array.
Currently, there are four kinds of commercially available VOA devices in the market, they are opto-mechanical VOA devices using stepping motor or magneto-optical crystal, and VOA devices based on waveguide technology, using liquid crystal (LC) technology, and using Micro-Electro-Mechanical Systems (MEMS) technology. The opto-mechanical VOAs are capable of providing consistent and stable attenuation by using stepping motor or magneto-optic crystal to drive a shutter or light blocker into a light beam to obstruct part or all of the light power. However, they can not be minimized to meet the needs of high channel-count integration due to the bulky size of the stepping motor or the electromagnetic coil. Essentially, the major drawbacks are their bulky size, long response time, difficulty of system integration and high cost. The waveguide and LC VOAs, while being suitable for high channel-count integration, are lack of consistent and stable attenuation expressed in the form of high insertion loss (IL), high Polarization Dependent Loss (PDL), high Polarization Mode Dispersion (PMD) and sensitivity to ambient temperature. The temperature sensitivity is caused by a differential coefficient of temperature change of the refractive index between the waveguide and/or LC material and connected glass fiber cores. The forth approach is the MEMS based VOA. It is known as a promising alternative to realize free space light attenuation with excellent device performances at the aspects such as, short response time, wavelength independence, protocol and bit rate independence, etc. Because the size and weight of MEMS elements are relatively small, therefore the energy required to drive the MEMS actuator, mirror, and structure is lower than the other approaches, in the other words; the power consumption of MEMS VOA is lower than the other alternatives. The package size of MEMS VOA device is comparatively smaller than the other alternatives, as well.
However, the associated IL and back-reflection loss (BR loss) of MEMS VOAs can not be easily perfected due to the existing interfaces among different optical elements according to its nature of free space optical operation scheme. In order to reduce IL and BR loss, it is necessary to have good optical alignment with respect to all optical elements in micrometer scale. The requirement of precise and dedicate assembly works among MEMS elements, optics, and fibers leads to higher manufacturing cost. The traditional MEMS VOA device adopted four sorts of device configurations. The first reported MEMS VOA devices leave a narrow air gap between two fiber ends to allow the insertion of a MEMS shutter into the axial light path, i.e., so-called in-line type VOA. In this approach the MEMS shutter and fibers are assembled on top of silicon substrate. The light attenuation range for VOA application is determined in terms of the relative position of MEMS shutter, where this in-plane position is controlled via force balance between spring force and force generated by micro-actuators. Thereby it can control relative amount of attenuation by blocking part of light beams. B. Barber, et al., “A fiber connectorized MEMS variable optical attenuator,” IEEE Photon. Technol. Lett., Vol. 10, No. 9, pp. 1262–1264, September 1998, and C. Marxer, et al., “A variable optical attenuator based on silicon micromechanics,” IEEE Photon. Technol. Lett., Vol. 11, No. 2, pp. 233–235, February 1999, describe MEMS VOA devices based on this in-line type device configuration.
The second sort of device configuration is using the micromachined reflective grating modular to be operated as a voltage controllable variable optical attenuator, disclosed by A. A. Godil et al, “Polarization independent grating modular,” U.S. Pat. No. 6,501,600, Dec. 31, 2002. Besides, combining optics and a tilted mirror (or rotational mirror) to be assembled in three-dimensional configuration is proposed to be an approach of making MEMS VOA devices by several groups, the attenuated light is controlled by changing the tilted angle of said mirror. These published literatures regarding to the tilted mirror based approach include B. M. Andersen, et al., “MEMS variable optical attenuator for DWDM optical amplifiers,” in proceedings of Optical fiber communication conference (OFC 2000), Vol. 2, p. 260–262, Mar. 7–10, 2000; K. C. Robinson, “Variable optical attenuator,” U.S. Pat. No. 6,137,941, Oct. 24, 2000; H. Toshiyoshi, et al, “A 5-volt operated MEMS variable optical attenuator,” in proceedings of TRANSDUCERS, 12th International Conference on Solid-State Sensors, Actuators and Microsystems, Vol. 2, pp. 1768–1771, Jun. 8–12, 2003; R. Wayne Fuchs, et al., “Micro-electro mechanical based optical attenuator,” U.S. Pat. No. 6,538,816, Mar. 25, 2003; B. J. Costello, et al., “Optical switch,” U.S. Pat. No. 6,628,856, Sep. 30, 2003; V. I. Vaganov, “VOA device and attenuation method with improved linearity,” U.S. Pat. No. 6,628,882, Sep. 30, 2003. But both of grating and tilted mirror types of MEMS VOA devices require dedicate three-dimensional (3D) assembly works and precisely alignment works among the MEMS elements, optics, and housing.
Again, the BR loss and PDL of the in-line type VOA device is normally higher than the values derived from the grating and tilted mirror based VOA devices. It is because that the scattering light is so easy to be coupled into the input fiber port due to the characteristics of in-line light path design. The forth type of so-called off-axis reflection type MEMS VOA device comprises off-axis aligned input and output fiber ports, and is using a reflective mirror, in which is capable of moving along with one in-plane axis, to change the reflected light beam path according to different mirror position, thereby the light attenuation value is determined in terms of the coupled reflected light intensity regarding to parallel shift of reflected light path, i.e., the attenuation value is controlled via the mirror position. This off-axis in-plane light path design can naturally reduce the back-reflection loss what we normally saw in the in-line type VOA devices. Moreover, in this off-axis in-plane reflection type MEMS VOA case, the optical fibers and optics can be aligned and assembled on the top of silicon substrate of MEMS chip in a two-dimensional (2D) arranged manner (planar type) with the aids of micromachined trenches and alignment marks, etc. The tedious and labor intensive works regarding to 3D assembly and alignment works can be got rid of. The relative technology has been reported and disclosed by several groups, including C. Lee, “Challenges in optical MEMS commercialization and MEMS foundry”, Oral presentation materials in IEEE/LEOS International Conf. on Optical MEMS 2002, Lugano, Switzerland, Aug. 20-23, 2002; C.-H. Kim, et al., “MEMS reflective type variable optical attenuator using off-axis misalignment” in proceeding of IEEE/LEOS International Conf. on Optical MEMS 2002, Lugano, Switzerland, Aug. 20–23, 2002; J. H. Lee, et al., “Variable optical attenuator,” U.S. Pat. No. 6,459,845, Oct. 1, 2002; C. Chen, et al., “Development and application of lateral comb drive actuator,” Jpn. J. Appl. Phys., vol. 42, Part 1, No. 6B, p. 4067–4073, June 2003; C. Chen, et al., “Novel VOA using in-plane reflective micromirror and off-axis light attenuation”, IEEE Communications Mag., Vol. 41, No. 8, pp. S16–S20, August 2003; A. Bashir, et al., “A MEMS-based VOA with very low PDL,” IEEE Photonics Technology Letters, Vol. 16, No. 4, p. 1047–1049, 2004.
It is also important for VOA devices to have low IL, low PDL, and low BR loss for practical applications. Combining the MEMS elements with micro-optics provides VOA devices a free-space light path design approach. This is a key way to make the light beam coming from input fiber become collimated beam shape thereby to gain in better optical performances. The larger collimated beam size, from several tens to hundreds of micrometers, will make better optical performance, and make the acceptable alignment tolerance higher. However, it will also lead to a requirement that the corresponding MEMS actuator has to be able to provide enough static displacement to let micromirror fully block the incoming light beam in the case of in-line type, or reflect the incoming light beam perfectly in the case of off-axis in-plane reflection type. Controversially the tilted mirror based MEMS VOA can more efficiently modify the coupled light intensity by changing the reflected light angle. Thus the corresponding actuation displacement, or actuator driving voltage, or actuator power consumption of the tilted mirror based MEMS VOA is less than the required values for VOA devices based on other approaches, although tilted mirror based MEMS VOA devices suffered with the difficulty of 3D assembly and alignment.
According to aforementioned functional requirements for VOA application, the desirable design of a VOA should be based on free space light attenuation, an efficient approach to achieve light attenuation regarding to actuator design and attenuation mechanism, and planar (2D in-plane) assembly and alignment works. Therefore, the present invention provides a new VOA device especially with emphasizing in fulfilling such design requirements.